Pseudo-noise carrier suppression/image rejection up and down converters

ABSTRACT

Pseudo-noise (PN) carrier suppression up-converter and pseudo-noise image sideband rejection up-converter and down-converter circuits are provided. The image sideband rejection circuits enable the use of single step up-conversion without the need for the high Q filters associated with two-step up-conversion architectures. For carrier suppression, the frequency spectrum of an input signal is PSK (BPSK or QPSK) spread by a PN signal and the spread signal is up-converted using a PSK (BPSK or QPSK) modulated carrier (local oscillator) modulated by the same PN sequence. In an up-converter, a carrier is QPSK modulated using independent PN sequences and the frequency spectrum of the input signal is QPSK spread using the same independent PN sequences whereby image sideband rejection results upon up-converting the QPSK spread signal in a mixer using the QPSK modulated carrier. In a down-converter, a carrier signal which is QPSK modulated by independent PN sequences is used to down-convert an input signal in a mixer and the resulting down-converted signal is QPSK spread using the same independent PN sequences whereby image sideband rejection occurs.

FIELD OF THE INVENTION

The invention relates to transceivers and, in particular, to radiofrequency (RF) circuitry providing improved signal up and downconversion.

BACKGROUND OF THE INVENTION

In the RF transmitter portion of a transceiver data is typicallyscrambled, encoded, and interleaved before being modulated and thenup-converted and amplified for transmission. The receiver portion of thetransceiver performs the reverse processes including down-converting theRF received signal to baseband.

Conventional single step (also referred to as direct carrier modulation)up-converters have been unable to achieve satisfactory suppression ofthe undesired image sideband which, as a result of the up-conversionprocess performed by a mixer, is generated in addition to the desiredsideband. Equally, such conventional single step up-converters have beenunable to achieve satisfactory suppression of any local-oscillator feedthrough.

Suppression of the image sideband can be achieved with the use of a highQ, off-chip RF filter; however to achieve low loss and stable frequency,this presents a high cost solution.

The use of two-step up-converters (a first one used to up-convert to anIF carrier and a second one used to up-convert to the RF carrier) is oneknown means of suppressing the image sideband while avoiding theforegoing requirement of single step up-converters for a high Q,off-chip RF filter. Upon a first up-conversion to an IF carrier theimage sideband is removed by means of a low Q filter. Then the desiredIF sideband is up-converted to an RF carrier and another low Q filter isused to suppress the resultant image sideband and any local oscillatorfeed through. Although the cut-off bandwidth of each of the filters usedin such IF and RF circuits is higher than that required for a singlestep up-converter, which makes these filters more easily realizable,they are nevertheless high cost, off-chip components the use of whichis, preferably, to be avoided.

Another known means of suppressing the image sideband is provided by theconventional image rejection up-converter which splits the basebandsignal into two quadrature components, each of which is individuallyup-converted with a quadrature local oscillator and then the resultingup-converted signals are combined. The undesired image sideband issuppressed in the resultant combined signal without need for an RFfilter. However, this conventional image rejection up-converter does notsuppress any local oscillator feed through and requires a high degree ofphase and amplitude balance in the two up-converters.

The foregoing disadvantages associated with conventional single step andtwo-step up-converters also apply to their corresponding single step andtwo-step down-converters.

Accordingly, there is a need for an improved up-converter to provideeffective and efficient suppression of any local oscillator feedthrough. In addition, there is a need for an improved up-converter anddown-converter to provide effective and efficient rejection of the imagesideband.

SUMMARY OF THE INVENTION

Pseudo-noise (PN) carrier suppression up-converter and pseudo-noiseimage sideband rejection up-converter and down-converter circuits areprovided by the invention claimed herein. Advantageously, the imagesideband rejection circuits enable the use of single step up-conversionwithout the need for the high Q filters associated with two-stepup-conversion architectures.

In accordance with the invention there are provided an up-converter andmethod for up-converting an input signal with a carrier signal wherebycarrier suppression is achieved in the up-converted signal. Theup-converter comprises means for phase shift keying (PSK) spreading thefrequency spectrum of the input signal by a pseudo-noise (PN) signal toproduce a spread spectrum input signal, means for phase shift keying(PSK) modulating the carrier signal by the PN signal to produce amodulated carrier signal and means for up-converting the spread spectruminput signal with the modulated carrier signal. The PSK modulation maybe biphase shift keying (BPSK) using one PN signal for modulation orquadrature phase shift keying (QPSK) using two independent PN signalsapplied to quadrature carrier signals. The carrier signal is preferablyproduced by a local oscillator and the means for up-converting comprisesa mixer. In the preferred embodiment, the input signal is digital andthe up-converter further comprises a digital-to-analog converter (DAC)for converting the digital input signal to analog before the spreadingmeans produces the spread spectrum input signal. Alternatively, the DACmay convert the spread spectrum input signal to analog before theup-converting means up-converts the spread spectrum input signal.

In accordance with a further aspect of the invention there are providedan up-converter and method for up-converting an input signal with acarrier signal whereby both carrier suppression and image sidebandrejection are achieved. The spreading of the input signal is quadraturephase shift keying (QPSK) spreading and the modulation of the carriersignal is quadrature phase shift keying (QPSK) modulating. The QPSKspreading is by two independent PN signals and the QPSK modulating is bythe same independent PN signals. The QPSK spreading comprises BPSKspreading the frequency spectrum of quadrature input signals by theindependent PN signals to produce a QPSK spread spectrum input signal,the quadrature input signals being produced from the input signal. TheQPSK modulating comprises BPSK modulating quadrature carrier signals bysame the independent PN signals to produce a QPSK modulated carriersignal, the quadrature carrier signals being produced from the carriersignal.

Also in accordance with the invention there are provided adown-converter and method for down-converting an input signal from acarrier signal. The down-converter comprises means for quadrature phaseshift keying (QPSK) modulating an oscillator signal corresponding to thecarrier signal by two independent PN signals to produce a QPSK modulatedoscillator signal. The QPSK modulating comprises BPSK modulating eachone of two quadrature oscillators signals produced from the oscillatorsignal with a different one of the independent PN signals. Means areprovided for down-converting the input signal with the QPSK modulatedoscillator signal (e.g. a mixer) to produce a down-converted inputsignal. Means for quadrature phase shift keying (QPSK) de-spreading thefrequency spectrum of the down-converted input signal by the independentPN signals to produce an un-spread, down-converted output signal. TheQPSK de-spreading may be performed by BPSK frequency spectrumde-spreading of parallel duplicate down-converted input signals by theindependent PN signals whereby the parallel input signals are producedfrom the down-converted input signal.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention are described in detail below withreference to the following drawings in which like references pertain tolike elements throughout:

FIG. 1 is a block diagram of a circuit architecture for a pseudo-noisecarrier suppression up-converter configured in accordance with oneembodiment of the present invention;

FIG. 2 is a block diagram of a circuit architecture for a pseudo-noisecarrier suppression/image rejection up-converter configured inaccordance with another embodiment of the present invention;

FIG. 3 is a block diagram of a circuit architecture for a pseudo-noisecarrier suppression/image rejection up-converter configured inaccordance with a further embodiment of the present invention;

FIG. 4 is a block diagram of a circuit architecture for a pseudo-noisecarrier suppression/image rejection up-converter configured inaccordance with a further embodiment of the present invention;

FIGS. 5( a), (b), (c) and (d) are phasor constellation diagrams providedto pictorial illustrate how the image sideband is converted from QPSK toBPSK to achieve image sideband rejection upon up-conversion inaccordance with the invention; FIGS. 5( a) and (b) illustrate the phasorconstellations for each of the desired and image sidebands of a QPSKspread baseband signal in the case of up-conversion by an unmodulatedlocal oscillator and FIGS. 5( c) and (d) illustrate the phasorconstellations for each of the desired and image sidebands of a QPSKspread baseband signal in the case of up-conversion by a QPSK modulatedlocal oscillator in accordance with the invention (it is to be notedthat, for purposes of clarity of illustration, no data modulation isshown in these drawings);

FIG. 6 is a block diagram of a circuit architecture for a pseudo-noisecarrier suppression/image rejection up-converter configured according tothat of FIG. 2 but driving two parallel channels (i.e. for up-convertingtwo modulated signal fragments);

FIG. 7 is a block diagram of a circuit architecture for a pseudo-noisecarrier suppression/image rejection up-converter configured according tothat of FIG. 4 but driving two parallel channels (i.e. for up-convertingtwo modulated signal fragments);

FIG. 8 is a block diagram of a circuit architecture for a pseudo-noiseimage rejection down-converter in accordance with another embodiment ofthe invention, wherein the circuitry is corresponding to theup-converter circuitry of FIG. 3 (i.e. to perform processes which arethe reverse thereof; and,

FIG. 9 is a block diagram of a circuit architecture for a pseudo-noiseimage rejection down-converter in accordance with a further embodimentof the invention, wherein the circuitry is corresponding to theup-converter circuitry of FIG. 4 (i.e. to perform processes which arethe reverse thereof).

DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS

The circuit architecture for a carrier suppression pseudo-noise (PN)up-converter in accordance with a first embodiment of the presentinvention is shown by FIG. 1. As shown, a binary phase shift keying(BPSK) modulation signal 10 is produced by BPSK modulating an oscillatorsignal 35 output from a local oscillator 30 and that modulatedoscillator signal 10 is input to a mixer 80 (i.e. instead of theunmodulated local oscillator (LO) signal 35 as in a conventionalup-converter circuit design). Thus, the oscillator signal 35 becomes thecarrier signal for the up-converted input signal 50. A PN generator 20produces a PN sequence 25 (e.g. at a rate of 80 Mega-chips per second)which is used by a BPSK modulator 40 to modulate the oscillator 30 andproduce the BPSK modulated oscillator signal 10. The input (baseband)signal 50 (e.g. DC to 10 MHz) is fed to a digital-to-analog converter(DAC) 70 and the analog signal 65 output therefrom is BPSK spread bymeans of a signal frequency spectrum spreader 60, being a multiplier inthe form of a mixer in the illustrated embodiment, using the same PNsequence 25 which modulates the oscillator signal 35. The resulting BPSKspread spectrum signal 75 is input to the mixer 80 (for example, aGilbert cell mixer is used in one preferred implementation of thisembodiment). If desired, for an alternative embodiment, the design shownby FIG. 1 may be modified so as to interchange the order of the DAC 70and multiplier 60.

Upon up-conversion, performed by the mixer 80, the BPSK spread spectrumassociated with the input signal cancels the BPSK modulation of thelocal oscillator and, therefore, the result is that the desiredmodulation is produced for the output (RF) signal 85. In addition, anylocal oscillator feed through is suppressed since the local oscillator30 is BPSK modulated and, thus, has a reduced power spectral density.Accordingly, this circuit architecture achieves carrier suppression butdoes not reject the image sideband.

The circuit architecture shown by FIG. 2 represents a variant of that ofFIG. 1. This circuitry achieves both carrier suppression and imagerejection, with the latter being achieved by incorporating the carriersuppression circuitry of FIG. 1 into the conventional image rejectionup-converter circuit configuration described above. The BPSK modulationsignal 10 is fed through a quadrature splitter 100 to produce twoquadrature modulation signals 12, 14, each of which has been BPSKmodulated with the PN sequence 25. In addition, two quadrature basebandsignals I (56) and Q (58) are produced by a real time quadraturesplitting operator 53 which calculates the sine and the cosine of thetime varying phase of the baseband signal (phasor) 50. Each quadraturebaseband signal 56,58 is fed to a digital-to-analog converter (DAC) 71,72 and the analog signals 68, 69 are BPSK spread by means of multipliers(mixers) 61, 62 using the same PN sequence 25. The resulting BPSK spreadspectrum signals 78, 79 are input to separate mixers 82, 81 and theresulting up-converted signals 86, 87 are combined by an in-phasecombiner 102 to produce an up-converted output signal 88. If desired,for an alternative embodiment, the design shown by FIG. 2 may bemodified so as to interchange the order of the DAC's 71, 72 andmultipliers 61, 62.

As for the circuitry of FIG. 1 the BPSK spread spectrums associated withthe baseband signals cancel the BPSK modulation of the local oscillatorquadrature signals and any local oscillator feed through is suppressedsince the local oscillator signals are BPSK modulated and, thus, havereduced power spectral density. Further, upon combining the quadratureup-converted signals, image rejection is achieved.

FIG. 3 illustrates an exemplary preferred circuit architecture forapplicant's novel pseudo-noise image rejection/carrier suppressionup-converter which, advantageously, achieves both image sidebandrejection and local oscillator feed through suppression. Two independentbinary phase shift keying (BPSK) modulated local oscillator signals17,19 are produced in mixers 102, 104 by BPSK modulating quadratureoscillator signals 13,15 using statistically independent PN sequences24, 26 generated by PN generators 21, 22. The BPSK modulated oscillator(carrier) signals 17, 19 are combined by a quadrature signal combiner106 and the resulting QPSK modulated carrier signal 108 is fed to amixer 116.

In addition, two quadrature baseband signals I (56) and Q (58) areproduced by a real time quadrature splitting operator 53 whichcalculates the sine and the cosine of the time varying phase of theinput signal (phasor) 50. Using signal frequency spectrum spreaders 61,62, being multipliers in the form of mixers in the illustratedembodiment, each quadrature input (baseband) signal 56,58 is BPSK spreadby an independent PN sequence 24 or 26, being the same PN sequences usedto modulate the quadrature oscillator signals 13, 15. The resulting BPSKspread spectrum signals 66, 67 are combined by a combiner 110 and theresulting QPSK spread spectrum signal 74 is fed to a digital-to-analogconverter (DAC) 112. The analog signal 114 output from DAC 112 is inputto a mixer 116 to produce an up-converted output signal 120.

Upon up-conversion by the single mixer 116, the QPSK spreading of thebaseband signal cancels the QPSK modulation of the local oscillator forthe desired sideband. However, for the image sideband, the phasecombination of the QPSK spreading and the QPSK modulation results in aBPSK modulated spectrum. Since the BPSK spreading occupies a muchbroader spectrum than the un-spread image sideband modulation, its powerspectral density is decreased and image sideband rejection is therebyachieved. The amount of the resulting image rejection produced by thiscircuitry is governed by the ratio of the PN chip rate and the basebandmodulation rate. In the illustrated preferred embodiment of FIG. 3 thePN chip rate is 80 Mega-chips per second and the baseband modulationrate is 10 MHz so a value of 8 corresponds to the amount of imagerejection which is achieved by this circuitry. As for the previousembodiments, the local oscillator feed through is also suppressed sincethe LO is QPSK modulated and this reduces its power spectral density.

FIG. 4 shows a circuit architecture for a carrier suppression/imagerejection pseudo-noise up-converter configured as a variant of that ofFIG. 3 in that image rejection is achieved by incorporating the carriersuppression/image rejection circuitry of FIG. 3 into the conventionalimage rejection up-converter circuit configuration described above. Thetwo quadrature oscillator (carrier) signals 13, 15 are each BPSKmodulated with an independent PN sequence 24, 26 and the quadrature BPSKmodulated oscillator signals are combined to produce a QPSK modulatedcarrier (oscillator) signal. As shown, in similar manner to thecircuitry of FIG. 2, the QPSK modulated oscillator signal is fed to aquadrature splitter to produce quadrature QPSK modulated oscillatorsignals for input to mixers 136, 138.

The two quadrature input (baseband) signals 56,58 are realized bycalculating the sine and the cosine of the time varying phase of theinput signal. As for the circuit of FIG. 3 the quadrature input signalsare BPSK spread with the independent PN sequences 24, 26 and the spreadspectrum signals are then combined to form a QPSK spread spectrum signal74. The QPSK spread spectrum signal 74 is fed through a quadrature phasesplitter 130 and the resulting two quadrature QPSK spread spectrumsignals are fed through DAC's 132, 134.

Upon up-conversion in separate mixers 136, 138, the QPSK spreading ofthe baseband signal cancels the QPSK modulation of the local oscillatorfor the desired sideband. However, for the image sideband, the phasecombination of the QPSK spreading and the QPSK modulation results in aBPSK modulated spectrum. Since the BPSK spreading occupies a muchbroader spectrum than the un-spread image sideband modulation, its powerspectral density is decreased and the amount of image rejection isgoverned by the ratio of the PN chip rate and the baseband modulationrate. Further image rejection is achieved upon combining the quadratureup-converted signals 140, 144 by a signal combiner component 146. As aresult, the RF output signal 150 benefits from image rejection by meansof both phase cancellation (i.e. performed by the conventional imagerejection circuitry as described with reference to FIG. 2) and thequadrature spread spectrum processing (i.e. as described with referenceto FIG. 3). As for each of the circuit embodiments of FIGS. 2 and 3 thelocal oscillator feed through is also suppressed since the LO is QPSKmodulated, thereby reducing its power spectral density.

FIGS. 5( a), (b), (c) and (d) are provided to pictorially show how theimage sideband is converted from QPSK to BPSK to achieve image rejectionupon up-conversion in accordance with the present invention. FIGS. 5( a)and (b) illustrate the phasor constellations produced for each of thedesired and image sidebands of a QPSK spread baseband signal when it isup-converted using a conventional (i.e. unmodulated) local oscillator.FIGS. 5( c) and (d) illustrate the corresponding phasor constellationsproduced for each sideband when a QPSK modulated local oscillator isused for up-converting an identically QPSK spread baseband signal (it isto be noted that, for purposes of clarity of illustration, no datamodulation is shown in these drawings).

In the case where a conventional local oscillator is used (see FIGS. 5(a) and (b)) the phasor locations for QPSK spreading states 2 and 4 areinterchanged for the desired and image sidebands but the phasorlocations for QPSK spreading states 1 and 3 are identical for eachsideband. (Note that such use of a conventional LO is not desirable andis shown only for the purpose of illustrating the different phasorlocations in the two sidebands.) On the other hand, in the case where aQPSK modulated local oscillator is used (see FIGS. 5( c) and (d)) all ofthe spreading states for the desired sideband collapse to the samephasor location, thereby removing the spreading. However, the spreadingstates for the image sideband collapse to two opposite phasor locationswhich corresponds to a residual BPSK spreading.

FIGS. 6 and 7 illustrate uses of the pseudo-noise carriersuppression/image rejection up-converter circuits of FIGS. 2 and 4,respectively, to up-convert baseband signals in parallel channels A–A′and B–B′. As will be seen from these figures, the BPSK or QPSK,respectively, modulated oscillator signals in these parallel channelscircuits are shared by up-conversion circuitry for each baseband signalof each channel.

The pseudo-noise image rejection down-converter illustrated by FIG. 8comprises circuitry which corresponds to that of the up-convertercircuitry of FIG. 3. As for the up-converter circuitry, thedown-conversion circuitry of FIG. 8 achieves image sideband rejectionwithout the need for high Q filters or two-step down-conversioncircuits. Here, as for the up-converter of FIG. 3, a quadrature phaseshift keying (QPSK) modulated local oscillator signal (the oscillatorsignal corresponding to the carrier signal of the input signal to bedown-converted) is used in place of the local oscillator signal used ina conventional down-converter. This is achieved by combining two binaryphase shift keying (BPSK) modulated quadrature oscillator signals 222,226, whereby the individual BPSK modulations on each quadratureoscillator signal 210, 220 are statistically independent PN sequences214, 216 generated by PN generators 200, 205.

Following down-conversion by a mixer 230 using the QPSK modulatedoscillator, and digitization by an ADC 245, the resulting QPSK spreaddown-converted signal 250 is separated into two parallel, identicalcomponent signals 252, 254 by a real time signal splitting operator 251and these signals are QPSK de-spread using the PN sequences 214, 216.Signals 270, 272 are combined in-quadrature by a real time signalcombining operator 274 to produce an output (baseband) signal 278. Thelocation of the ADC is not critical and can be located following therecombination of the parallel, down-converted signals. For analternative embodiment (not shown) the down-converted signal 250 can beQPSK de-spread by first generating two quadrature phase down-convertedsignals using a quadrature splitter and then QPSK de-spreading theseusing the independent PN sequences and an in-phase combiner.

The QPSK de-spreading of the down-converted signal cancels the QPSKmodulation of the local oscillator for the desired sideband. However,for the image sideband, the phase combination of the QPSK de-spreadingand the QPSK modulation results in a BPSK modulated spectrum. Since theBPSK spreading occupies a much broader spectrum than the un-spread imagesideband modulation, its power spectral density is decreased. The amountof image rejection is governed by the ratio of the PN chip rate and thebaseband modulation rate.

Another embodiment of a quadrature pseudo-noise image rejectiondown-converter is illustrated by FIG. 9 which comprises circuitrycorresponding to that of the up-converter circuitry of FIG. 4. As shown,the two quadrature oscillator signals 300, 305 are each BPSK modulatedwith independent PN sequences 310, 315 and the BPSK modulated oscillatorsignals are combined to produce a QPSK modulated oscillator signal. TheQPSK modulated oscillator signal is fed through a quadrature splitter,as shown, to provide quadrature QPSK modulated oscillator signals forinput to mixers 330, 335.

The input signal 320 to be down-converted is split into quadraturesignals 322, 324 and down-converted by mixers 330, 335. The twoquadrature analog down-converted (baseband) signals 340,342 aredigitized by ADC's and combined in-quadrature by a real time signalcombining operator 350 to produce a spread spectrum down-convertedsignal 355 which is then separated into two identical component signalsby a real time signal splitting operator 360. The quadrature digitaldown-converted signals 365, 370 are then BPSK de-spread using the PNsequences 310, 315. The BPSK de-spreading of these down-convertedsignals are combined into QPSK de-spreading by the quadrature combiner375.

The QPSK de-spreading cancels the QPSK modulation of the localoscillator for the desired sideband. However, for the image sideband,the phase combination of the QPSK de-spreading and the QPSK modulationresults in a BPSK modulated spectrum. Since the BPSK spreading occupiesa much broader spectrum than the un-spread image sideband modulation,its power spectral density is decreased. The amount of image rejectionis governed by the ratio of the PN chip rate and the baseband modulationrate. Further image rejection is achieved upon combining the quadraturedown-converted (baseband) signals by means of the quadrature combiner375. Therefore, as for the up-converter of FIG. 4, image rejection isachieved through both phase cancellation and spread spectrum techniques.

The individual electronic and processing functions utilised in theforegoing described embodiments are, individually, well understood bythose skilled in the art. It is to be understood by the reader that avariety of other implementations may be devised by skilled persons forsubstitution. Persons skilled in the field of communication design willbe readily able to apply the present inventions to an appropriateimplementation method for a given application.

Consequently, it is to be understood that the particular embodimentsshown and described herein by way of illustration is not intended tolimit the scope of the inventions claimed by the inventor which aredefined by the appended claims.

1. A down-converter for down-converting an input signal from a carriersignal, said down-converter comprising: (a) means for quadrature phaseshift keying (QPSK) modulating oscillator signals by independent PNsignals to produce a QPSK modulated carrier signal, said oscillatorsignals being produced from an oscillator signal corresponding to saidcarrier signal; (b) means for down-converting said input signal withsaid modulated oscillator signal to produce a down-converted inputsignal; and, (c) means for quadrature phase shift keying (QPSK)de-spreading the frequency spectrum of down-converted input signals bysaid independent PN signals to produce an un-spread, down-convertedoutput signal, said down-converted input signals being produced fromsaid down-converted input signal.
 2. A down-converter according to claim1 wherein said QPSK de-spreading comprises BPSK frequency spectrumde-spreading parallel, duplicate down-converted signals by saidindependent PN signals and combining the resulting de-spread signals inquadrature and said QPSK modulating comprises BPSK modulating quadraturecarrier signals with said independent PN signals.
 3. A down-converteraccording to claim 1 wherein said QPSK de-spreading comprises BPSKfrequency spectrum de-spreading quadrature down-converted signals bysaid independent PN signals and combining the resulting de-spreadsignals in-phase and said QPSK modulating comprises BPSK modulatingquadrature carrier signals with said independent PN signals.
 4. Adown-converter according to claim 3 wherein said means fordown-converting comprises a mixer.
 5. A down-converter according toclaim 4 wherein said input signal is analog and said down-converterfurther comprises an analog-to-digital converter (ADC).
 6. Up-conversionapparatus for up-converting a first signal to a second signal with acarrier signal, said apparatus comprising: (a) a local oscillatorconfigured for generating a carrier signal; (b) a quadrature phasesplitter configured for producing quadrature carrier signals from saidcarrier signal; (c) at least two pseudorandom signal generators eachconfigured for generating an independent pseudorandom signal; (d) atleast two BPSK modulators each configured for BPSK modulating adifferent one of said quadrature carrier signals with a different one ofsaid independent pseudorandom signals; (e) a combiner configured forcombining BPSK modulated quadrature carrier signals output from two saidBPSK modulators to produce a QPSK modulated carrier signal; (f) aquadrature phase splitter configured for producing quadrature signalsfrom said first signal; (g) at least two signal frequency spectrumspreaders each configured for producing a BPSK spread spectrum signal ofa different one of said quadrature signals produced from said firstsignal with a different one of said independent pseudorandom signals;(h) a combiner configured for combining BPSK spread spectrum signalsoutput from two said spreaders to produce a QPSK spread spectrum signal;(i) an up-converter configured for up-converting said QPSK spreadspectrum signal with said QPSK modulated carrier signal. 7.Up-conversion apparatus according to claim 6 wherein said up-convertercomprises a mixer.
 8. Down-conversion apparatus for down-converting afirst signal on a carrier signal to a second signal, said apparatuscomprising: (a) a local oscillator configured for generating anoscillator signal corresponding to said carrier signal; (b) a quadraturephase splitter configured for producing quadrature oscillator signalsfrom said oscillator signal; (c) at least two pseudorandom signalgenerators each configured for generating an independent pseudorandomsignal; (d) at least two BPSK modulators each configured for BPSKmodulating a different one of said quadrature oscillator signals with adifferent one of said independent pseudorandom signals; (e) a combinerconfigured for combining BPSK modulated quadrature oscillator signalsoutput from two said BPSK modulators to produce a QPSK modulatedoscillator signal; (f) a down-converter configured for down-convertingsaid first signal with said QPSK modulated oscillator signal to producea down-converted first signal; (g) a signal splitter configured forproducing parallel, identical down-converted signals from saiddown-converted first signal; (h) at least two signal frequency spectrumde-spreaders each configured for producing a BPSK de-spread signal of adifferent one of said parallel signals with a different one of saidindependent pseudorandom signals; and, (i) a quadrature combinerconfigured for combining in-quadrature said BPSK de-spread signals. 9.Down-conversion apparatus according to claim 8 wherein saiddown-converter comprises a mixer.
 10. A method for down-converting aninput signal from a carrier signal, said method comprising: (a)quadrature phase shift keying (QPSK) modulating an oscillator signal bytwo independent pseudo-noise (PN) signals to produce a QPSK modulatedoscillator signal, said oscillator signal corresponding to said carriersignal and said QPSK modulating comprising BPSK modulating each of twoquadrature oscillator signals produced from said oscillator signal by adifferent one of said independent PN signals; (b) down-converting saidinput signal with said modulated oscillator signal; and, (c) quadraturephase shift keying (QPSK) spreading the frequency spectrum of saiddown-converted input signal by said two independent pseudo-noise (PN)signals to produce an output signal of which an image sideband hasreduced power spectral density from that of a desired sideband thereof,said QPSK spreading comprising BPSK spreading the frequency spectrum ofeach of two down-converted input signals produced from saiddown-converted input signal by a different one of said independent PNsignals.
 11. A method according to claim 10 whereby said down-convertingcomprises multiplying said input signal and said modulated oscillatorsignal.